Water resources are under major stress around the world. Rivers, lakes, and underground aquifers supply fresh water for irrigation, drinking, and sanitation, while the oceans provide habitat for a large share of the planet's food supply. Today, however, expansion of agriculture, damming, diversion, over-use, and pollution threaten these irreplaceable resources in many parts of the globe.

Providing safe drinking water for the more than 1 billion people who currently lack it is one of the greatest public health challenges facing national governments today. In many developing countries, safe water, free of pathogens and other contaminants, is unavailable to much of the population, and water contamination remains a concern even for developed countries with good water supplies and advanced treatment systems. And over-development, especially in coastal regions and areas with strained water supplies, is leading many regions to seek water from more and more distant sources (Fig. 1).

This unit describes how the world's water supply is allocated between major reserves such as oceans, ice caps, and groundwater. It then looks more closely at how groundwater behaves and how scientists analyze this critical resource. After noting which parts of the world are currently straining their available water supplies, or will do so in the next several decades, we examine the problems posed by salinization, pollution, and water-related diseases.

Scientists widely predict that global climate change will have profound impacts on the hydrologic cycle, and that in many cases these effects will make existing water challenges worse. As we will see in detail in Unit 12, "Earth's Changing Climate," rising global temperatures will alter rainfall patterns, making them stronger in some regions and weaker in others, and may make storms more frequent and severe in some areas of the world. Warming will also affect other aspects of the water cycle by reducing the size of glaciers, snowpacks, and polar ice caps and changing rates of evaporation and transpiration. In sum, climate change is likely to make many of the water-management challenges that are outlined in this unit even more complex than they are today.

At the same time, many current trends in water supply and water quality in Europe and North America are positive. Thirty years ago, many water bodies in developed countries were highly polluted. For example, on June 22, 1969, the Cuyahoga River in Cleveland, Ohio, caught fire when sparks ignited an oily slick of industrial chemicals on its surface. Today, the United States and western European countries have reduced pollution discharges into rivers and lakes, often producing quick improvements in water quality. These gains show that when societies make water quality a priority, many polluted sources can be made usable once again. Furthermore, in the United States water consumption rates have consistently declined over the last several decades.